Sign up to receive free email alerts when patent applications with chosen keywords are publishedSIGN UP

Abstract:

Nanocatalysts and methods of synthesizing and using the same are
provided.

Claims:

1. A nanoparticle comprising a mesoporous silica particle and Au
nanoparticles, wherein said Au nanoparticles are contained within the
mesopores of said mesoporous silica particle, and wherein the surface of
the mesopores of the mesoporous silica particle comprises a reducing
agent.

2. The nanoparticle of claim 1, wherein said mesoporous silica is SBA-15.

3. The nanoparticle of claim 1, wherein said reducing agent is a
hemiaminal group.

4. The nanoparticle of claim 1, wherein said reducing agent is an imine
group.

5. The nanoparticle of claim 1, wherein said Au nanoparticles have a
diameter of about 3 nm to about 10 nm.

7. The nanoparticle of claim 6, wherein said capping group is a methyl
group.

8. The nanoparticle of claim 6, wherein said capping group is an n-alkyl
group.

9. A method of synthesizing the nanoparticle of claim 1 comprising
contacting mesoporous silica particles with oxidized Au, wherein the
surface of the mesopores of the mesoporous silica particle comprises a
reducing agent.

10. The method of claim 9, wherein said reducing agent is a hemiaminal
group or an imine group.

11. The method of claim 9, wherein said oxidized gold is Au(III) or
Au(I).

12. The method of claim 9, further comprising synthesizing said
mesoporous silica particles by a) synthesizing silica particles in the
presence of a surfactant, b) grafting the external surface of the silica
particles with capping groups, c) removing the surfactant, and d)
functionalizing the mesopores with a reducing agent.

13. A method of catalyzing a chemical reaction, said method comprising
adding at least one nanoparticle of claim 1 to said chemical reaction.

14. The method of claim 13, wherein said chemical reaction is an
oxidation reaction.

Description:

[0001] This application claims priority under 35 U.S.C. §119(e) to
U.S. Provisional Patent Application No. 61/484,040, filed on May 9, 2011.
The foregoing application is incorporated by reference herein.

FIELD OF THE INVENTION

[0003] The present invention relates to the field of catalysts.
Specifically, efficient and selective nanocatalysts, methods of
synthesis, and methods of use thereof are disclosed.

BACKGROUND OF THE INVENTION

[0004] Several publications and patent documents are cited throughout the
specification in order to describe the state of the art to which this
invention pertains. Each of these citations is incorporated herein by
reference as though set forth in full.

[0005] The use of metal nanoparticles (MNPs) in catalysis has rapidly
increased in recent years because of their efficient and intrinsic
size-dependent catalytic properties as well as their ability to catalyze
a range of chemical reactions (Nishihata et al. (2002) Nature
418:164-167; Astruc et al. (2005) Angew. Chem., Int. Ed., 44:7852-7872;
Moreno-Manas et al. (2003) Acc. Chem. Res., 36:638-643; Li et al. (2002)
Langmuir 18:4921-4925; Ranu et al. (2009) Pure Appl. Chem., 81:2337-2354;
Barbaro et al. (2010) Dalton Trans., 39:8391-8402; Migowski et al. (2006)
Chem. Eur. J., 1:32-39; Durand et al. (2008) Eur. J. Inorg. Chem.,
23:3577-3586). For many MNPs to catalyze reactions or result in efficient
catalysis, the reacting substrates must directly interact with the metal
surfaces. This metal-substrate interaction would be greater if the MNPs
were synthesized "naked". Unfortunately, however, atoms of "naked" MNPs
have a greater tendency to aggregate into a bulk material due to their
high surface energies, which results in loss of, or decrease in, their
intrinsic catalytic activity and selectivity over time (Moulijn, et al.
(2001) Appl. Catal. A: Gen., 212:3-16; Xing et al. (2007) Chem. Mater.,
19:4820-4826). In particular, Pd nanoparticles (PdNPs), which are well
known for their catalytic activities, can easily aggregate to form
Pd-black because of the very high surface energy of palladium (Iwasawa et
al. (2004) J. Am. Chem. Soc., 126:6554-6555). Although the degree of
aggregation of PdNP or other MNP catalysts can be overcome or minimized
by passivating the metals' surfaces with organic ligands, this too will,
unfortunately, be accompanied by the loss of catalytic activity because
the very sites on the metals where catalysis takes place will be covered
by these surface passivating organic groups (Jayamurugan et al. (2009) J.
Mol. Catal. A: Chem., 307:142-148). Accordingly, there is a strong need
for efficient and recyclable nanocatalysts.

SUMMARY OF THE INVENTION

[0006] In accordance with the present invention, catalytically active
nanoparticles are provided. In a particular embodiment, the nanoparticle
comprises a mesoporous silica particle and Au nanoparticles. In a
particular embodiment, the Au nanoparticles are contained within the
mesopores of the silica particle, which are functionalized with a
reducing agent (e.g., a hemiaminal or imine group). The nanoparticles may
also comprise capping groups (e.g., methyl or alkyl groups) on the
external surface of the silica particle.

[0007] In accordance with another aspect of the instant invention, methods
of synthesizing the nanoparticle are provided. In a particular
embodiment, the method comprises contacting mesoporous silica particles
with oxidized Au (e.g., Au(III) or Au(I)), wherein the surface of the
mesopores of the mesoporous silica particle are functionalized with a
reducing agent. The method may further comprise synthesizing the
mesoporous silica particles. In a particular embodiment, the mesoporous
silica particles are synthesized by a) synthesizing silica particles in
the presence of a surfactant, b) grafting the external surface of the
silica particles with capping groups, c) removing the surfactant, and d)
functionalizing the mesopores with a reducing agent.

[0008] In accordance with yet another aspect of the instant invention,
methods of catalyzing a chemical reaction are provided. In a particular
embodiment, the method comprises adding at least one nanoparticle of the
instant invention to the chemical reaction. In a particular embodiment,
the chemical reaction is an oxidation reaction, particularly one that
leads to the ketone formation on an alkane (e.g., linear alkane or aryl
substituted alkane). In a particular embodiment, the method comprises
performing multiple rounds (e.g., 2 or more, 3 or more, etc.) of the
chemical reaction with the same catalytic nanoparticles.

BRIEF DESCRIPTIONS OF THE DRAWING

[0009] FIG. 1 provides transmission electron microscope (TEM) images of
Au/SBA-15 catalysts (a) A, (b) B and (c) C that were prepared from 0.01,
0.1 and 1.0 mM, respectively, of aqueous HAuCl4 solutions. Graphs of
the average size of the Au nanoparticles are also provided.

[0010] FIG. 2 provides 13C CP-MAS NMR spectra of as-synthesized
SBA-15 material whose external surface has been functionalized with
methyl (-Me groups), before (Bottom) and after (Top) calcination at
350° C. The calcination step removed the P123 groups, leaving the
Me groups, which are visible in the Top spectrum.

[0011]FIG. 3 provides transmission electron microscopy (TEM) images of
Au/SBA-15 nanocatalysts that were prepared from SBA-15 material
containing no Me groups on its external surface. 0.5, 1.0 and 2.0 mM of
HAuCl4 solutions were used to produce these materials labelled as
(a) A', (b) B' and (c) C', respectively.

[0020] FIG. 12 provides GC-MS spectra of the three ketones (A)
2-hexadecanone, (B) 3-hexadecanone, and (C) 4-hexadecanone produced from
the oxidation reaction of n-hexadecane catalyzed by our Au/SBA-15
catalyst.

[0021]FIG. 13 provides a mechanism for Au/SBA-15 catalyzed oxidation of
alkane (ethylbenzene) into a predominant ketone product with a minor
alcohol products and some t-BuOH by-product.

DETAILED DESCRIPTION OF THE INVENTION

[0022] Herein, the synthesis of nanoporous silica supported gold
nanoparticle catalysts is reported along with their efficient catalytic
activities in oxidation of various substituted alkylbenzenes and linear
alkanes. The Au nanoparticles are synthesized by reducing Au(III) ions in
situ within the nanopores of hemiaminal-functionalized mesoporous silica
using the hemiaminal groups as reducing agents. The resulting mesoporous
silica-supported gold nanoparticles efficiently catalyze the oxidation
reactions of various alkyl-substituted benzenes and linear alkanes using
t-butyl hydroperoxide (TBHP) as an oxidant. The catalytic reactions yield
up to ˜99% reactant conversion and up to ˜100% selectivity to
ketone products in some cases. This high selectivity to ketone products
by the catalysts is unprecedented, especially considering the fact that
it is achieved under mild reaction conditions and without using any
additives in the reaction mixture. In the case of n-hexadecane oxidation,
the catalytic reactions generate no alcohol byproducts, unlike other
similar catalytic systems that are recently reported in the literature.
Recyclability and leaching tests for the catalyst are also included. The
possible reaction mechanism for this Au-nanoparticle catalyzed alkane
oxidation with TBHP oxidant into a ketone product with some minor alcohol
byproducts is proposed. The reactions leading to the products appear to
take place through two major steps, involving several radical
intermediates that lead to a ketone and an alcohol.

[0023] Oxidative catalysis is an important route for the synthesis of many
commodity chemicals as well as perfumes, drugs and pharmaceuticals (Caron
et al. (2006) Chem. Rev., 106:2943-2989). In particular the oxidation of
alkanes, which are relatively more abundantly available, produces a
number of more valuable commodity chemicals. However, alkane oxidation
still remains to be one of the most difficult reactions to perform
because it involves harsh reaction conditions, or it requires corrosive
chemical reagents such as potassium permanganate, potassium dichromate or
ammonium cerium nitrate (Punniyamurthy et al. (2005) Chem. Rev.,
105:2329-2363; Sheldon et al., Green Chemistry and Catalysis. (Wiley-VCH
Verlag GmbH & Co KgaA, Weinheim.) 2007; Clerici et al. (1998) Catal.
Today, 41:351-364). Thus, currently one of the major efforts in oxidative
catalysis research is finding or developing effective alkane oxidation
catalysts that can efficiently catalyze the oxidation of alkanes through
activation of their C--H bonds. The second major effort in oxidative
catalysis is the design and development of catalysts that can operate
under mild reaction conditions and generate selectively the desired
product.

[0024] Among the various alkanes used as substrates in alkane oxidation
reactions, linear and phenyl-substituted alkanes such as ethylbenzene
stand out as among the most important ones because their oxidation
products are essential precursors for many types of pharmaceuticals and
synthetic materials. For instance, the oxidation products of ethylbenzene
such as acetophenone and 1-phenylethanol are useful as precursors in the
synthesis of optically active alcohols, benzalacetophenones (chalcones)
and hydrazones (Mehler et al. (1994) Tetrahedron Asym., 5:185-188;
Blickenstaff et al. (1995) Bioorg. Med. Chem., 3:917-922; Newkomeand et
al. (1966) J. Org. Chem., 31:677-681).

[0025] Current industrial practice of ethylbenzene oxidation is performed
via thermal autoxidation, in the absence of any catalyst, often to
produce ethylbenzene hydroperoxide, among other things. Furthermore, to
date very few catalysts have been explored for oxidation of ethylbenzene
(Ma et al. (2007) Catal. Lett., 113:104-108; Lu et al. (2010) J. Mol.
Catal. A: Chem., 331:106-111; Toribio et al. (2009) Appl. Catal. A: Gen.,
363:32-39; Orli ska, B. (2010) Tetrahedron Lett., 51:4100-4102; Shilov et
al. (1997) Chem. Rev., 97:2879-2932). Furthermore, these previously
reported catalysts have relatively weak catalytic activities in
ethylbenzene oxidation, or they require comparatively harsh reaction
conditions. For instance, mesoporous silica functionalized with
cobalt(II) oxide (Co/SBA-15) was reported to catalyze the oxidation of
ethylbenzene; however, the catalytic reaction was shown to work only at
relatively high temperatures (120 to 150° C.), giving only
moderate % conversion of ethylbenzene--the highest reported value being
70.1% in 9 hours at 150° C. In addition, this catalyst was
reported to form mixed uncontrolled oxidation products such as
1-phenylethyl hydroperoxide, benzoic acid, acetophenone and
1-phenylethanol (Ma et al. (2007) Catal. Lett., 113:104-108). In another
example, the oxidation of ethylbenzene with hydrogen peroxide as an
oxidant was shown to be catalyzed by the homogeneous catalyst
8-quinolinolato manganese(III) complex (Lu et al. (2010) J. Mol. Catal.
A: Chem., 331:106-111). However, this catalytic reaction was also
reported to give very small (26%) conversion of ethylbenzene, even after
using ammonium acetate and acetic acid as additives in the reaction
mixture. In another report, the oxidation of ethylbenzene into its
hydroperoxide was achieved under soft reaction conditions in air in the
presence of N-hydroxyimides such as N-hydroxysuccinimide,
N-hydroxymaleinimide or N-hydroxynaphthalimide (Toribio et al. (2009)
Appl. Catal. A: Gen., 363:32-39). Furthermore, the yield of the reaction
to a specific product, that is, peroxyethyl benzene, was shown to improve
by the addition of a minute amount of sodium hydroxide into the reaction
mixture. However, the product selectivity of this catalytic reaction is
still less efficient to be utilized for many industrial applications.
Improvement of the selectivity of alkane oxidations can be improved by
using different compounds as additives. For instance, in Cu(II), Co(II)
or Mn(II) salt-catalyzed oxidation reactions of isopropylaromatic
compounds to their corresponding alcohol or ketone products, including
acetophenone, the use of N-hydroxyphthalimide as an additive was shown to
improve the reaction's selectivity (Orli ska, B. (2010) Tetrahedron
Lett., 51:4100-4102). However, the use of additives to improve the
selectivity of oxidative reactions makes the catalytic system more
costly.

[0026] Besides these aforementioned metal salts, a few others transition
metals in homogeneous form were also reported to catalyze the oxidation
of various hydrocarbons, including alkylbenzene (Shilov et al. (1997)
Chem. Rev., 97:2879-2932). Interestingly, gold, both in the form of metal
complexes as well as in the form of nanomaterials, has increasingly
become attractive in recent years for use as a catalyst for a broad range
of oxidative catalytic reactions. For example, both Au(I) and Au(III)
complexes have been successfully used as homogeneous catalysts for alkane
oxidations (Shulpin et al. (2001) Tetrahedron Lett., 42:7253-7256).

[0028] Despite these aforethentioned reports on Au nanoparticle-based
catalysts and catalysis, the oxidative catalysis of alkyl-substituted
benzenes and the selective oxidation of n-alkanes to ketone products
efficiently by nanosized Au particles have not been demonstrated before.
Ketones are versatile functional groups in organic chemistry and key
intermediates for a number of products (Blickenstaff et al. (1995)
Bioorg. Med. Chem., 3:917-922; Newkomeand et al. (1966) J. Org. Chem.,
31:677-681). Thus their synthesis selectively in high yield from
oxidation of alkanes would be tremendously important in various chemical
processes. Furthermore, most of the previous reports on oxidative
catalysis by Au nanoparticles have focused on alkene and alcohol
substrates, which are relatively easier to oxidize than alkanes. In
addition, many of the previously reported Au nanocatalysts were shown to
work either under extreme conditions or catalyze reactions into a mixture
of products consisting of alcohols, acids, aldehydes and ketones (Corma
et al. (2008) Chem. Soc. Rev., 37:2096-2126; Chen et al. (2009) J. Am.
Chem. Soc., 131:914-915; Wu et al. (2010) Microporous Mesoporous Mater.,
141:222-230).

[0029] Herein, the synthesis and efficient catalytic activity of
mesoporous silica supported-nanosized Au particles for oxidation of
alkanes is reported, both in the form of alkyl-substituted benzenes and
n-alkanes, using an oxidant such as TBHP. The mesoporous silica supported
Au nanoparticles were prepared by an in situ hemiaminal reduction method.
The Au nanoparticles were shown to efficiently catalyze the oxidation of
various phenyl-substituted alkanes including ethylbenzene, as well as
linear alkanes such as n-hexane and n-hexadecane at lower temperature
(70° C.), producing selectively carbonyl (ketone) products.

[0030] The instant invention provides nanoparticles that with unexpectedly
superior properties. The nanoparticles provided herein are efficient
catalysts, exhibit high selectivity, and are recyclable without the loss
of catalytic activity. The mesoporous silica supported Au nanoparticles
and methods of synthesis are described in more detail hereinbelow. In a
particular embodiment, the mesoporous silica supported Au nanoparticles
of the instant invention have a diameter of less than about 1000 nm, less
than about 750 nm, or less than about 500 nm. Compositions comprising at
least one mesoporous silica supported Au nanoparticles of the instant
invention and at least one carrier are also encompassed by the instant
invention.

[0031] The nanoparticles of the instant invention comprise mesoporous
silica, corrugated/nanoporous core-shell silica (e.g., etched (e.g., by
KOH) silica microspheres; see, e.g., silica constructs of U.S. patent
application Ser. No. 13/396,052), and/or porous titania (e.g., mesoporous
titania) particles encompassing Au nanoparticles. While the instant
application generally refers to mesoporous silica, these other silica and
titania particles may be used in place of the mesoporous silica. The term
"mesoporous" indicates that the material contains pores with diameters
between about 2 and about 50 nm. In a particular embodiment, the
mesoporous silica particles have pores with diameters from about 2 to
about 25 nm, about 5 to about 25 nm, about 2 to about 15 nm, or about 5
nm to about 10 or 12 nm. In a particular embodiment, the mesoporous
silica particles are generally spherical. Types of mesoporous silica
include, without limitation, MCM- (e.g., MCM-41, MCM-48), SBA- (e.g.,
SBA-15, SBA-1, SBA-16), MSU- (e.g., MSU-X, MSU-F), KSW- (e.g., KSW-2),
FSM- (e.g., FSM-16), HMM- (e.g., HMM-33), and TUD (e.g., TUD-1). In a
particular embodiment, the mesoporous silica is SBA-15. In a particular
embodiment, the mesoporous material wall thickness is about 0.5 to about
10 nm, about 1 to about 7 nm, or about 1.5 to about 6 nm. With regard to
the core-shell nanospheres, the shells may range from about 2 to about 60
nm in thickness, particularly about 4 to about 40 nm in thickness. The
cores of the core-shell nanospheres may range from about 50 to about 600
nm, particularly about 100 to 450 nm in diameter.

[0032] In a particular embodiment, the mesoporous silica of the
nanoparticles of the instant invention comprises capping groups on their
external surface. The capping group may be an alkyl capping group.
Examples of capping groups include, without limitation, methyl, n-propyl,
n-pentyl, and n-octadecyl groups. The mesoporous silica of the
nanoparticles of the instant invention may also comprise a reducing agent
(e.g., a mild reducing agent) attached to the mesopore channel surface.
In a particular embodiment, the reducing agent is a hemiaminal group
(i.e., a functional group that comprises a hydroxyl group and an amine
attached to the same carbon atom (C(OH)(NR2), wherein R is H or
alkyl). The reducing agent may also be an imine. In a particular
embodiment, the imine is a functional group comprising the structure
R3--N═C(R1)R2, wherein R1, R2, and R3
are independently H or alkyl. In a particularly embodiment, the imine
comprises the
structure--(CH2)n--N═CH--(CH2)m--CH3. In a
particular embodiment, n is about 1 to about 10 or about 1 to about 6 and
m is about 0 to about 7 or about 0 to about 4.

[0033] The Au nanoparticles within the mesoporous silica particles may
have a diameter small enough to fit within the mesopores. In a particular
embodiment, the Au nanoparticles have a diameter from about 2 to about 25
nm, about 2 to about 15 nm, about 3 to about 10, or about 5 nm to about
10 nm. In a particular embodiment, the Au is Au(0) within the mesoporous
silica particles.

[0035] The instant invention also encompasses methods of catalyzing a
chemical reaction with the nanoparticles described herein. The
nanoparticles may be used to catalyze, for example, oxidation reactions.
In a particular embodiment, the nanoparticles are used to catalyze
oxidation reactions of alkanes, including linear alkanes and
aryl-substituted alkanes. The nanoparticles of the instant invention
selectively catalyze the formation of ketones. In a particular
embodiment, the reaction is performed in the presence of an oxidant,
particularly TBHP. The reaction may be performed in any appropriate
solvent, particularly a polar, aprotic solvent. Furthermore, based on the
recyclable properties of the nanoparticles of the instant invention,
methods comprising multiple rounds of chemical reactions without the need
to replace or re-charge the catalyst are encompassed herein.

DEFINITIONS

[0036] The following definitions are provided to facilitate an
understanding of the present invention:

[0037] As used herein, the term "catalyst" refers to a substance that
increases the rate of a chemical reaction while not being consumed in the
reaction.

[0038] As used herein, the term "selective" refers to the capability of
the catalyst to cause the production of specific products by selectively
catalyzing a specific reaction, particularly in a mixture of similarly
reactive compounds or from competitive reactions.

[0039] As used herein, the term "turnover number" refers to the number of
moles of reactant that a mole of catalyst can convert to product before
becoming inactivated.

[0041] The term "alkane" includes straight and branched chain
hydrocarbons. Typically, an alkane will comprise 1 to about 20 carbons or
1 to about 10 carbons in the main chain. The hydrocarbon chain of the
alkane may be interrupted with one or more oxygen, nitrogen, or sulfur.
The alkane may, optionally, be substituted (e.g., with 1 to 4
substituents). Substituents include, without limitation, alkyl, alkenyl,
halo (such as F, Cl, Br, I), haloalkyl (e.g., CCl3 or CF3),
alkoxyl, alkylthio, hydroxy, methoxy, carboxyl, oxo, epoxy,
alkyloxycarbonyl, alkylcarbonyloxy, amino, carbamoyl (e.g.,
NH2C(═O)-- or NHRC(═O)--, wherein R is an alkyl), urea
(--NHCONH2), alkylurea, aryl, ether, ester, thioester, nitrile,
nitro, amide, carbonyl, carboxylate and thiol.

[0042] The term "aryl," as employed herein, refers to monocyclic and
bicyclic aromatic groups containing 6 to 10 carbons in the ring portion.
Examples of aryl groups include, without limitation, phenyl, naphthyl,
such as 1-naphthyl and 2-naphthyl, indolyl, and pyridyl, such as
3-pyridyl and 4-pyridyl. Aryl groups may be optionally substituted
through available carbon atoms, preferably with 1 to about 4 groups.
Exemplary substituents are described above. The aryl groups may be
interrupted with one or more oxygen, nitrogen, or sulfur heteroatom ring
members (e.g., a heteroaryl).

[0043] The following example provides illustrative methods of practicing
the instant invention, and is not intended to limit the scope of the
invention in any way.

[0045] SBA-15 was synthesized following the original procedure with a
minor modification (Zhao et al. (1998) Science 279:548-552; Xie et al.
(2008) J. Phys. Chem. C, 112:9996-10003). A solution of 12 g of
Pluronic®-123 ((PEO)2O(PPO)70(PEO)20, 313 g Millipore
water and 72 g HCl (˜36 wt %) was prepared and stirred vigorously
at 40° C. until all the Pluronic®-123 was dissolved. After
adding 25.6 g of TEOS, the solution was stirred at 45° C. for 24
hours. The solution was then kept under static conditions at 80°
C. in oven for another 24 hours to age. The resulting reaction mixture
was filtered, and the precipitate was washed with copious amount of water
and dried under ambient conditions. This produced as-synthesized SBA-15
mesostructured material. To graft the external surface of the
mesostructured material with methyl groups, 4.0 g of this as-synthesized
SBA-15 was suspended in a solution containing 30 mL of
hexamethyldisilazane (HMDS) and 300 mL of toluene. The solution was then
mildly stirred for 8 hours at room temperature in order to functionalize
the external Si--OH groups of the as-synthesized SBA-15 with
--Si(CH3)3 (or -Me) groups, and prevent possible growth of
bigger metal nanoparticles on the outer surface of the mesoporous
material from the reduction of Au(III) ions in the solution. The solid
sample was recovered by filtration, washed with toluene and ethanol
(2×10 mL in each case), and then let to dry under ambient
conditions. This resulted SBA-15 sample with Me functional groups on its
outer surface. It was then calcined in tube furnace at 350° C. for
5 hours under the flow of air to remove the Pluronics® template from
its mesopores, without touching the Me groups (Xie et al. (2008) J. Phys.
Chem. C, 112:9996-10003; Sun et al. (2006) J. Am. Chem. Soc.,
128:15756-15764). The resulting mesoporous SBA-15, which was capped with
external methyl groups and had free silanol groups in its mesopores, was
denoted as Me-SBA-15.

Synthesis of Hemiaminal-Functionalized SBA-15 (Hemiaminal-SBA-15)

[0046] The Me-SBA-15 synthesized above was dried in an oven for 12 hours
at 80° C. before being grafted with amine groups. 1.5 g of the
well-dried Me-SBA-15 was stirred in a solution of 4.5 mL of
3-amonopropyltriethoxysilane (APTS) in 120 mL of toluene for 6 hours at
80° C. to graft its mesoporous channel surface with primary amine
groups. After filtration and washing with anhydrous ethanol (2×10
mL), the resulting sample (labelled as NH2--SBA-15) was left to dry
under ambient conditions. The NH2--SBA-15 (1 g) material was then
suspended in a mixture of 20 mL of ethanol and 10 mL of 37.2%
formaldehyde solution at 40° C. for 1 hour. This produced a white
colored, hemiaminal-functionalized mesoporous silica sample, denoted here
as Hemiaminal-SBA-15.

In-Situ Synthesis of Au Nanoparticles within the Pores of
Hemiaminal-SBA-15 (Au/SBA-5)

[0047] For the in-situ synthesis of Au nanoparticles within the mesoporous
silica material, 50 mg of Hemiaminal-SBA-15 was dispersed in 10 mL of
three different concentrations (0.01, 0.1 and 1.0 mM) of aqueous
HAuCl4 in a mixture of ethanol and water (1:4) and stirred for 30
minutes at 80° C. The resulting Au/SBA-15 samples, labelled as A,
B, and C, respectively, were separated by filtration, washed with 20 mL
water and then 10 mL ethanol, and let to dry under ambient conditions.

Catalytic Oxidation Reaction

[0048] The catalytic oxidation reactions were carried out in a 50 mL three
neck round bottom flask. In a typical oxidation reaction, 1 mmol alkane
substrate, 15 mg Au/SBA-15 catalyst, 2 mmol (TBHP or H2O2)
oxidant and 0.5 mL of chlorobenzene as an internal standard were mixed
with a solvent (see Tables for different solvents used). The reaction was
stirred with a magnetic stirrer. Samples were withdrawn after intervals
of time and analyzed by gas chromatography (GC) and gas
chromatography-mass spectrometry (GC-MS).

Materials and Catalyst Characterizations

[0049] Nitrogen gas adsorption/desorption isotherms of all the mesoporous
materials and catalysts were performed on Micromeritics TriStar® 3000
volumetric adsorption analyzer after degassing the samples at 160°
C. for 12 hours. Thermogravimetric traces were collected on a TA Q50
Analyzer with a temperature ramping rate of 10° C./minute from
room temperature to 780° C. under nitrogen gas flow. The UV-Vis
absorption spectra of the Au/SBA-15 samples were measured with a Lambda
850 spectrophotometer (PerkinElmer; Waltham, Mass.). For the diffuse
reflectance spectra measurement, the mesoporous powder samples containing
the gold nanomaterials were sandwiched between two 3×3 cm quartz
slides. Powder X-ray diffraction (XRD) patterns were recorded on a
Siemens, Daco-Mp instrument having Cu--Kα radiation with wavelength
of 1.54 Å. The diffractometer was set to 40 kV accelerating voltage
and 30 mA. The XRD data were obtained by setting a wide scan range of 29
from 20° to 80° with step size of 0.015° and dwell
time of 5 seconds. Transmission electron microscopy (TEM) images were
obtained with a TOPCON microscope operated at 200 KV. The samples were
prepared first by dispersing them in ethanol, casting a drop of the
solution carbon/formvar coated Cu grids and letting them dry. The
catalytic reactions were probed by withdrawing reaction mixtures in
intervals of time and analyzing them by gas chromatography (GC) using an
Agilent 6850 GC equipped with an HP-1 column (1% dimethyl polysiloxane,
30 m length, 0.25 mm internal diameter, 0.25 μm film thickness) and a
flame ionization detector. The products were further confirmed by gas
chromatography-mass spectrometry (GC-MS) (HP-5971) that was equipped with
an HP-5 MS 50 m×0.200 mm×0.33 μm capillary column.

Synthesis of Reference Au/SBA-15 Material from SBA-15, whose External
Surface is not Passivated by Me Groups

[0051] SBA-15 material was prepared following the same procedure as
reported (Zhao et al. (1998) Science 279:548-552; Xie et al. (2008) J.
Phys. Chem. C, 112:9996-10003). A solution of 12 g of Pluronics®-123
((PEO)20(PPO)70(PEO)20, 313 g millipore water and 72 g HCl
(˜36 wt %) was prepared and stirred vigorously at 40° C.
until all the Pluronic®-123 was dissolved. Then, 25.6 g of TEOS was
added into the solution and it was stirred at 45° C. for 24 hours.
After this, the solution was kept under static conditions at 80°
C. in oven for another 24 hours to age. The resulting reaction mixture
was filtered, and the precipitate was washed with copious amount of water
and dried under ambient conditions. This produced as-synthesized SBA-15
mesostructured material. The solid sample was recovered by filtration,
washed with toluene and ethanol, and then let to dry under ambient
conditions. This externally functionalized SBA-15 sample was calcined in
tube furnace at 550° C. for 5 hours under the flow of air to
remove the Pluronics® template, resulting in mesoporous SBA-15 with
no organic capping groups on its external surface.

[0052] The SBA-15 sample was dried in an oven for 12 hours at 80°
C. before being grafted with amine groups. Typically, 1.5 g of well-dried
SBA-15 was stirred in a solution of 4.5 mL of
3-amonopropyltriethoxysilane (APTS) in 120 mL of toluene for 6 hours at
80° C. to graft its mesoporous channel surface with primary amine
groups. After filtration and washing with anhydrous ethanol (2×10
mL), the resulting sample NH2-functionalized SBA-15 was left to dry
under ambient conditions and then let to react with 37.2% formaldehyde
solution in anhydrous ethanol at 40° C. for 1 hour. This produced
Hemiaminal-functionalized SBA-15.

[0053] This materials was then immobilized with Au(III) ions for the
in-situ synthesis of Au nanoparticles within the SBA-15 mesoporous silica
material. Typically, 50 mg of the Hemiaminal-functionalized SBA-15 was
dispersed in 10 mL of three different concentrations (0.5, 1.0 and 2.0
mM) of aqueous HAuCl4 in a mixture of ethanol and water (1:4) and
stirred for 30 minutes at 80° C. The resulting Au/SBA-15 samples,
labelled as A', B', and C', respectively, were separated by filtration,
washed with 20 mL water and then 10 mL ethanol, and let to dry under
ambient conditions.

[0054] This attempted synthesis of the Au nanoparticles using an SBA-15
material that does not have Me groups on its external surface also
resulted in Au nanoparticles; however, as can be seen in FIG. 3, the
sizes of the Au nanoparticles at higher concentrations were bigger than
the size of the channel of the mesoporous silica. This shows the
importance of placing organic capping groups on the external surface of
the SBA-15 to prevent possible growth of the Au nanoparticles on the
outside surface. Their corresponding N2 gas adsorption isotherms and
pore size distributions are also shown in FIG. 5.

[0055] Since the sizes of most of the gold nanoparticles in sample C'
(FIG. 3C) are in the range of 8-14 nm and they appear to be bigger than
the size of the mesopores of SBA-15 (˜9 nm). Thus, many of these
particles should be outside the pores of the materials. Careful
observation of the TEM images confirmed that this was the case. The Au
nanoparticles may have been formed inside the pores of the mesoporous
silica first, as in samples A' and B', but then diffused out as their
sizes grew further because of the relatively larger concentration of
HAuCl4 used for the preparation of C'. Nonetheless, this sample, C',
with its bigger Au nanoparticles outside the mesopores allows for the
investigation of the effect of size of Au nanoparticles in the oxidation
reaction.

Results

Synthesis and Characterization

[0056] FIG. 1 displays transmission electron microscope (TEM) images of
mesoporous silica-supported Au nanoparticles. The nanoparticles were
synthesized by reducing Au(III) ions with hemiaminal groups that were
tethered onto the mesopore channel surface of SBA-15 mesoporous silica
(Xie et al. (2008) J. Phys. Chem. C, 112:9996-10003). To achieve this,
first SBA-15 mesostructured silica was prepared (Zhao et al. (1998)
Science 279:548-552; Xie et al. (2008) J. Phys. Chem. C, 112:9996-10003).
Before surfactant extraction, the outer surface of the SBA-15
mesostructured silica was functionalized with methyl (-Me) groups using
hexamethyldisilazane (HMDS). This produced mesostructured SBA-15 silica
containing -Me groups on its outer surface. The removal of the surfactant
templates at moderate temperature of 350° C. from the material
resulted in reactive silanol (Si--OH) groups within its inner channel
pores while leaving the -Me groups on the outer surface (FIG. 2) (Xie et
al. (2008) J. Phys. Chem. C, 112:9996-10003; Sun et al. (2006) J. Am.
Chem. Soc., 128:15756-15764). The temperature of 350° C. is chosen
for calcination of the material because the Pluronics® templates
undergo decomposition at this temperature, but not the -Me groups (Xie et
al. (2008) J. Phys. Chem. C, 112:9996-10003; Sun et al. (2006) J. Am.
Chem. Soc., 128:15756-15764). The resulting material was labelled as
Me-SBA-15. This step allows for formation of small and more monodisperse
Au nanoparticles, predominantly within the channel pores of the SBA-15.
As shown in the TEM images in FIG. 3, without this step, bigger Au
nanoparticles could form. After calcination, the hydroxyl groups formed
in the mesopores of Me-SBA-15 were let to react with
3-aminopropyltriethoxysilane (APTS), and form 3-aminopropyl groups (or
--NH2 groups), only within the mesopores of the material. The
--NH2 groups in the resulting sample, denoted as NH2--SBA-15,
were then let to react with formaldehyde, and form surface grafted
hemiaminal groups. The resulting sample was labelled as
Hemiaminal-SBA-15.

[0057] Upon addition of Au(III) solution into Hemiaminal-SBA-15, large
numbers of reasonably monodisperse Au nanoparticles within the channel
pores of the material were formed from the reaction between the
hemiaminal groups and the Au(III) ions. This resulted in the Au/SBA-15
samples (or Au/SBA-15 nanocatalysts). Three different concentrations,
that is, 0.01, 0.1 and 1.0 mM, aqueous solutions of HAuCl4 were
stirred with 50 mg of Hemiaminal-SBA-15 for 30 minutes at 80° C.
This produced three different Au/SBA-15 samples having different sized Au
nanoparticles in them. The samples were labelled as A, B and C,
respectively.

[0058] The Au/SBA-15 samples and their parent materials, including
Me-SBA-15, NH2--SBA-15 and Hemiaminal-SBA-15, were characterized by
various spectroscopic and analytical methods. The N2 gas adsorption
measurements showed a type-IV isotherm for all the samples, indicating
the presence of mesoporous structures in all the Au/SBA-15 samples as
well as their parent materials (FIG. 4). For comparison purposes the
N2 gas adsorption isotherms of the reference Au/SBA-15 materials
prepared from SBA-15, whose external surface is not capped with -Me
groups, are also included in FIG. 5. The pore diameter of SBA-15 material
before deposition of Au nanoparticles had monodisperse pore sizes with
average Barret-Joyner-Halenda (BJH) pore diameter of ˜8.0 nm.
Similarly, the pore size distributions of all the mesoporous materials
after deposition of Au nanoparticles showed the presence of reasonably
monodisperse mesopores; however, the average pore sizes decreased
slightly to average BJH pore diameters with values ranging between 5.7 to
6.1 nm, presumably because the bigger pores had been filled with the
deposited Au nanoparticles: The Brunauer-Emmett-Teller (BET) surface
areas of Me-SBA-15, NH2--SBA-15 and Hemiaminal-SBA-15 were 829, 449
and 348 m2g-1, respectively. This indicates that there is a
decrease in surface area as more organic groups are immobilized within
the pores of the materials, as expected. The BET surface areas of the
Au/SBA-15 samples A, B and C were 388, 381, and 373 m2g-1,
respectively.

[0059] The thermogravimetric analysis (TGA) for the mesoporous samples
Me-SBA-15, NH2--SBA-15 and Hemiaminal-SBA-15 showed a weight loss
below 100° C., which was attributed to the loss of physisorbed
water (FIG. 6). In the temperature range of 100-550° C., the TGA
traces showed weight losses of 3.4, 7.0, and 9.9% for samples Me-SBA-15,
NH2--SBA-15, and Hemiaminal-SBA-15, respectively. These weight
reductions in the range of 100-550° C. were mainly due to the loss
of methyl, organoamine, and/or hemiaminal groups from the samples upon
heating. It can also be noted that the weight loss in the range of
100-550° C. from NH2--SBA-15 was more than twice that from
Me-SBA-15. This indicates the presence of more organic groups in the
former due to the presence of both 3-aminopropyl and methyl groups in it.
Similarly, the significantly higher weight loss from Hemiaminal-SBA-15
compared to that from NH2--SBA-15 in the same temperature range was
an indirect indication of the presence of the bulkier hemiaminal groups
in place of the --NH2 groups. The presence of all the different
organic groups were further confirmed by elemental analyses and by
13C CP MAS NMR spectra shown in FIG. 2 (Xie et al. (2008) J. Phys.
Chem. C, 112:9996-10003; Sun et al. (2006) J. Am. Chem. Soc.,
128:15756-15764). Careful calculations based on the differences in weight
losses seen in TGA for the different samples indicated that there were
˜0.97 mmol hemiaminal groups/g of Hemiaminal-SBA-15.

[0060] Further analysis by TEM (FIG. 1) showed the presence of reasonably
monodisperse Au nanoparticles with average particle sizes of 5.4
(±1.2), 6.9 (±1.7) and 8.4 (±2.3) nm for Au/SBA-15 samples A, B
and C, respectively. The formation of these different Au/SBA-15 samples
has allowed us to investigate the effect of size of Au nanoparticles on
their catalytic activities in alkane oxidation reactions (see below). The
TEM images in FIG. 1 and the TEM images of the parent materials (for
instance, of Me-SBA-15's that is shown in FIG. 7) also exhibit that the
materials have well-ordered mesoporous channels.

[0061] The formation of Au nanoparticles in the samples was further
confirmed by diffuse UV-Vis spectroscopy by using the powdered Au/SBA-15
materials A, B, and C as samples (FIG. 8). The characteristic plasmon
bands corresponding to Au nanoparticles were observed at ˜521 nm
for samples A and B, but at ˜525 nm for sample C. The more
blue-shifted absorption maxima for A and B compared to that of C
indicates that the size of Au nanoparticles in samples A and B were
slightly smaller than that in sample C, which is in agreement with the
TEM results. In addition, powder X-ray diffraction (XRD) was used to
characterize the Au nanoparticles of Au/SBA-15 samples (FIG. 9). The XRD
patterns of all the Au/SBA-15 samples showed Bragg reflections at
2θ values of 38, 44, 64 and 77°. These Bragg reflections
were indexed as the (111), (200), (220) and (311) diffracting planes,
respectively, of metallic Au (Yan et al. (2005) Catal. Commun.,
6:404-408). Careful inspection of the full-width-at-half-maxima (FWHM) of
the Bragg reflections at 20 of 38° on the XRD patterns indicated
that the peaks were slightly broader for A compared to B and C. This
indicates that the Au nanoparticles in A were slightly smaller in size
than those in B and C, which is consistent with the results obtained by
TEM and UV-Vis analyses.

[0062] The presence of Au in the samples was further corroborated by
ICP-AES which showed the presence of 1.08, 3.86 and 4.56 wt % Au in
samples A, B and C, respectively. This corresponds to 54.8, 196.0, and
231.5 μmol Au/g of Au/SBA-15 (or 0.055, 0.196, and 0.232 mmol Au/g of
Au/SBA-15). This clearly shows that the mol % of Au increases in the
order of A<B<C, which is consistent with the amount of Au(III) used
in the syntheses. However, the difference in wt % of Au produced between
samples B and C was relatively smaller than that between A and B,
although the corresponding difference in the mol of Au(III) used in the
syntheses was the same. This is most likely due to the limited number of
hemiaminals (or the reducing agents) present in the Hemiaminal-SBA-15
sample, causing a larger fraction of the Au(III) ions in case of C to
remain unreduced. It is worth noting that 0.01, 0.10, and 1.00 mM
concentrations of aqueous HAuCl4 solution with 10 mL volume were
used for 50 mg Hemiaminal-SBA-15 to synthesize Au/SBA-15 samples A, B and
C, respectively. This implies that 0.1, 0.2, and 0.4 mmol Au(III),
respectively, were used per gram of hemiaminal-SBA-15 sample. On the
other hand, the hemiaminal-SBA-15 has a constant amount, 0.97 mmol,
hemiaminals (or reducing agents) per gram of hemiaminal-SBA-15. Since
three hemiaminals are required to reduce one Au(III) ion into Au(0), the
0.97 mmol hemiaminals/gram of hemiaminal-SBA-15 would be capable of
reducing the theoretical maximum of only 0.32 mmol Au(III) into Au(0).
This means, the amount of Au(III) used in case of C was slightly more
than the available hemiaminal reducing agents in the hemiaminal-SBA-15
sample, which resulted in more incomplete reduction of the Au(III) used
for during the synthesis of sample C. In fact, the filtrate from the
synthesis of sample C was found to contain much more Au(III) than that in
sample C or sample C by ICP-AES analysis.

Catalytic Properties

[0063] The catalytic activities of the synthesized Au/SBA-15 materials A,
B, and C were then tested in alkane oxidation reactions under similar
conditions. Ethylbenzene, various other alkyl-substituted benzenes,
n-hexane and n-hexadecane were used as model substrates. Ethylbenzene was
chosen as a model substrate because its oxidation using Au nanoparticles
as catalysts has never been reported previously. Furthermore, as
mentioned above, the oxidation products from ethylbenzene are important
precursors for a number of useful products (Mehler et al. (1994)
Tetrahedron Asym., 5:185-188; Blickenstaff et al. (1995) Bioorg. Med.
Chem., 3:917-922; Newkomeand et al. (1966) J. Org. Chem., 31:677-681).
The other alkyl-substituted benzenes as well as n-hexane and n-hexadecane
were used as substrates in order to demonstrate the versatility of the
catalysts, investigate the scope of the catalytic reaction, and also
evaluate the catalytic activity/selectivity of Au/SBA-15 with respect to
other catalysts (Dapurkar et al. (2009) Catal Lett., 130:42-47).

[0064] As discussed above, the synthesis of Au/SBA-15 using different
concentrations of HAuCl4 produces samples containing different sized
Au nanoparticles (or the samples labelled here as A, B and C). By using
these three different Au/SBA-15 samples as catalysts, the effect of the
size of the Au nanoparticles of Au/SBA-15 on their catalytic activities
in the oxidation of alkanes, specifically alkylbenzene, was studied.
Furthermore, the catalytic oxidation of ethylbenzene by Au/SBA-15 was
investigated with the three commonly used oxidizing agents: air
(O2), H2O2 and TBHP (Table 1). Attempted oxidation of
ethylbenzene with catalyst B in air, or with oxygen bubbled into the
reaction mixture, as oxidant at 70 or 110° C. did not yield any
oxidation product. Attempted oxidation of the reaction mixture, even at
300 Psig pressure of air in a Parr reactor, did not generate any
oxidation product. When H2O2 was also used as an oxidant with
the Au/SBA-15 catalyst, again no oxidation of ethylbenzene took place.
The lack of oxidation with air or H2O2 was likely due to the
use of larger Au nanoparticles. However, when TBHP was used as an
oxidant, Au/SBA-15 was able to catalyze the oxidation of ethylbenzene
efficiently. Furthermore, in a control experiment with SBA-15 and TBHP,
ethylbenzene did not undergo any oxidation in 36 hours. Thus, the
presence of Au/SBA-15 as catalyst and TBHP as oxidant allowed for
ethylbenzene to undergo oxidation, indicating that the alkane oxidation
reaction is catalyzed by the Au nanoparticles and with TBHP oxidant.

[0065] It is worth noting, however, that TBHP itself can undergo some Au
nanoparticle-catalyzed decomposition into t-BuOH. In fact, in the control
experiment, where Au/SBA-15 catalyst B and TBHP are mixed, with no
ethylbenzene, 11% of the TBHP was decomposed into t-BuOH. Thus, two
equivalents of TBHP was used in the catalytic reactions in order to make
up for any possible decomposition of TBHP and to ensure the presence of
enough TBHP in the reaction mixture.

Effect of Gold Nanoparticle Size on Catalytic Efficiency

[0066] Because TBHP was successfully served as an oxidant for ethylbenzene
oxidation in the presence of our Au/SBA-15 catalyst, it was used in
further studies below. For instance, in the presence of two equivalent of
TBHP as oxidant, catalyst A, which contained 5.4±1.2 nm Au
nanoparticles, resulted in 57% conversion of ethylbenzene, and gave a
high selectivity (89%) to acetophenone product, with a minor (11%)
1-phenylethanol byproduct (Table 2 and FIG. 9). Au/SBA-15 catalysts B and
C, which consisted of 6.9±1.7 and 8.4±2.3 Au nanoparticles,
respectively, generated 79 and 89% conversions of ethylbenzene, with 93
and 94% selectivities, respectively, to acetophenone product. The
remaining 6-7% byproduct was again the secondary alcohol 1-phenylethanol
in both cases.

[0067] Based on these results, one can conclude that catalyst C has a
better catalytic efficiency and selectivity to a ketone product than
catalysts A and B; and catalyst B, in turn, has better catalytic
efficiency than catalyst A. However, when comparing the results based
catalytic turn-over-numbers (TONs) and turn-over-frequencies (TOFs)
(Table 1), an opposite trend in catalytic efficiencies was observed. That
is, catalyst A gives significantly higher TON and TOF (764 and 23
h-1) than catalyst B (274 and 8 h-1); and sample B, in turn,
gives higher TON and TOF than catalyst C (256 and 7 h-1). This
indicates that among the three different Au/SBA-15 catalysts studied for
oxidation of ethylbenzene, the catalytic activity of A was actually
greater than that of B or C when the catalytic activities were compared
on the basis of catalytic activity per mol of Au. Because not all the
supported Au nanoparticles such as those in the middle of mesopores, and
because not all the atoms of the Au nanoparticles such as those in middle
of the nanoparticles are exposed to participate in the catalytic
reactions, the reported catalytic TONs and TOFs per total mol of Au are
underestimations of the TON and TOFs.

[0068] The results in Table 2 also clearly indicate that the catalytic
activities of Au nanoparticles of Au/SBA-15 catalysts in ethylbenzene
oxidation vary with the size of the nanoparticles. This is also
consistent with results in previous reports for other reactions involving
oxidation of various organic substrates is shown to depend on the size of
Au nanoparticles (Chen et al. (2009) J. Am. Chem. Soc., 131:914-915;
Haider et al. (2008) Catal. Lett., 125:169-176; Chen et al. (2004)
Science 306:252-255; Chen et al. (2006) Catal. Today, 111:22-33). For
instance, the rate as well the selectivity of the Au nanoparticles in
alcohol oxidation reactions are shown to be affected by the size of Au
nanoparticles, with 6.9 nm-sized Au particles yielding the highest
catalytic efficiency (Hudlicy, T. (2010) SYNLETT., 18:2701-2707). On the
other hand, a smaller size (3.5-4.0 nm) Au nanoparticles are found to be
the most effective in gas phase oxidation reactions, particularly CO
oxidation (Chen et al. (2004) Science 306:252-255; Chen et al. (2006)
Catal. Today, 111:22-33).

[0069] As shown in Table 3, the type of solvent used in ethylbenzene
oxidation in the presence of Au/SBA-15 catalyst affects both the
catalytic efficiency as well as the catalytic selectivity of the
reaction. This was tested using different solvents with catalyst B at
70° C. for 36 hours. In the case of acetonitrile, 79% conversion
of ethylbenzene with 93% selectivity to acetophenone product was obtained
(Table 3, entry 1). When tetrahydrofuran (THF) was used as the solvent, a
lower ethylbenzene conversion of 70% and a lower selectivity of 87% to
acetophenone product were obtained (Table 3, entry 2). Upon using
ethylacetate as the solvent, further decrease in the catalytic activity
as well as selectivity to acetophenone resulted (Table 3, entry 3). In
the case of toluene as the solvent (Table 3, entry 4), even lower
catalytic activity and lower selectivity were obtained. This decrease in
the catalytic activity of Au/SBA-15 in oxidation reaction in the order of
acetonitrile>THF>ethylacetate>toluene might be the result of the
lower degree of solubility of the reaction intermediates during the
oxidation reactions in solvents with decreasing polarity or dielectric
constant (Andrade et al. (2005) Current Org. Chem., 9:195-218).
Generally, the dipolar, aprotic solvents such as THF gave better results
than the non-polar solvents such as toluene. Most importantly, the reason
that certain solvents work better than others seems to suggest that some
of the solvents may undergo co-oxidation and produce a more oxidizing
agent in the reaction. It has been previously reported that solvents such
as methylcyclohexene work much better than any other solvent to form a
peroxyl radical, which is then epoxidizing the substrate stylbene.
Acetonitrile is known to produce peroxycarboximidic acid--a powerful
oxidation agent. THF is also very well known to undergo radical
oxidation. Hence, the `good` solvents are probably speeding up the
formation of radicals, or more importantly speeding up the chain length
of the radicals, making them more available for the oxidation of the
substrate ethylbenzene.

[0070] Since among all the solvents tested, acetonitrile resulted in the
highest % conversion of ethylbenzene while giving the highest selectivity
toward a particular product--in this case, a ketone (or acetophenone,
1)--all the other reactions for the subsequent studies (discussed below)
were performed in acetonitrile.

[0071] For instance, Au/SBA-15 catalyst B catalyzed the oxidation of
1,3-diethylbenzene with 80% conversion and 88% selectivity to
3-ethylacetophenone product in 36 hours. Interestingly, the catalytic
oxidation of 1,3-diethylbenzene with Au/SBA-15 stopped after the
oxidation of only one of its ethyl groups, and with no formation of
1,3-diacetophenone product in 36 hours (Table 4, entry 2). Au/SBA-15
sample B also catalyzed the oxidation of diphenylmethane with 99%
conversion, and a remarkably high selectivity of ˜100% to
benzophenone product (Table 4, entry 3). Furthermore, Au/SBA-15 catalyzed
the oxidation of propylbenzene with 75% conversion and 95% selectivity to
propiophenone, with 5% 1-phenyl-2-propanol byproduct (Table 4, entry 4).

[0072] When n-hexane was used as a substrate, catalyst B oxidized it with
95% conversion in 8 hours, giving 92% 2-hexanone as a major product
(Table 4, entry 5). However, when this reaction was further continued for
30 hours, all the 2-hexanone was further converted into 2,4-di-hexanone
product, and without resulting any alcohol or acid byproducts.

Catalytic Properties on n-alkanes

[0073] To fully compare the relative catalytic activities of Au/SBA-15
materials with these previous reports, additional study of oxidation
n-hexadecane using Au/SBA-15 as catalyst was performed (Table 5). While
the Au/SBA-15 catalyzed also the oxidation of n-hexadecane,
interestingly, it gave exclusively ketone products, with no alcohol or
other oxidized products (Chen et al. (2009) J. Am. Chem. Soc.,
131:914-915). Furthermore, the catalytic selectivity of Au/SBA-15
catalyst to particular ketone products was much higher. For instance,
Au/SBA-15 catalyst B gave only two or three ketone products, namely
2-hexadecanone and 4-hexadecanone (sometimes with a minor 3-hexadecanone
product, depending on the reaction conditions) (Table 5). These products
are confirmed by GC and GC-MS which are shown in FIGS. 10 and 11.

[0074] Without being bound by theory, this significant catalytic
selectivity shown by the instant Au/SBA-15 in n-hexadecane oxidation
might be due to three reasons: 1) the size of Au nanoparticle in
Au/SBA-15 were higher than that in Chen et al.; 2) the supported Au
nanoparticles in Au/SBA-15 do not have strongly bound alkanethiol ligands
around them or are `naked`; and (3) the difference in the type of
oxidants employed in the two cases. Although oxygen, a greener oxidant,
was successfully used by Chen et al., it gave a mixture of seven
different ketones and six different alcohols (Chen et al. (2009) J. Am.
Chem. Soc., 131:914-915). On the other hand, TBHP, which is a less
`greener` oxidant, was employed herein, but it gave much more selective
products consisting of only two or three different ketones, with no
alcohol byproduct.

[0075] The products obtained from the catalytic oxidation of n-hexadecane
using Au/SBA-15 catalyst were rather similar to those reported for
catalytic ozonation of n-hexadecane by activated charcoal or 0.5% Pd-,
Ni-, and V-loaded microporous ZSM-5 catalysts (Rajasekhar et al. (2009)
Ind. Eng. Chem. Res., 48:9097-9105). In the latter case, only three
ketones, that is, 4-hexadecanone, 3-hexadecanone and 2-hexadecanone were
reported. In addition, their results showed that 4-hexadecanone was the
major product while 3-hexadecanone and 2-hexadecanone were produced in
roughly the same, but less significant, amounts. Herein, 3-hexadecanone
was sometimes not observed at all, depending on the reaction conditions,
while either 4-hexadecanone or 2-hexadecanone was formed as major
products (see Table 5).

[0076] The recyclability of the Au/SBA-15 catalyst for multiple uses in
alkane oxidations was also studied (Table 6). The catalytic selectivity
of Au/SBA-15 to produce ketone products remained unchanged or still very
high, even after the catalyst was recycled a few times. However, its
catalytic activity showed significant reduction, especially after the
third cycle. Nevertheless, the amount of Au leached into the solution was
very minimal, as characterized by ICP-AES analysis. For instance, after
three reaction cycles, the amount of Au in the sample decreased from 196
to 192 μmol Au per gram of Au/SBA-15 (B). In addition, the amount of
Au in the reaction mixture was obtained to be only ˜409 ppm
(˜3.84 μmol). Thus, the loss of the catalytic activity of the
catalyst is probably mainly due to the inactivation or possible pore
clogging of the mesoporous channels of the material.

[0077] Without being bound by theory, FIG. 13 provides a mechanism for the
Au/SBA-15 catalyzed oxidation reaction. The oxidant TBHP is well known to
undergo radical chemistry (Barton et al. (1998) New J. Chem., 22:565-568;
Barton et al. (1998) New J. Chem., 22:559-563; Barton et al. (1998)
Tetrahedron 54:15457-15468; Liu et al. (2010) Chem. Commun., 46:550-552;
Li, Y. F. (2007) SYNLETT., 2922-2923; Mendez et al. (2010) Dalton Trans.,
39:8457-8463; Mitsudome et al. (2009) Adv. Synth. Catal., 351:1890-1896).
In experiments involving addition of a radical scavenger TEMPO in the
middle of the reaction, the alkane oxidative reaction in the presence of
the Au/SBA-15 catalyst stopped immediately. This suggested that the
Au/SBA-15-catalyzed reaction, not surprisingly, goes through radical
intermediates. Notably, some TBHP underwent decomposition into t-BuOH in
the presence of Au/SBA-15 in the control experiment. Furthermore,
catalyst B in the presence of two equivalent of TBHP as oxidant was found
to form 79% conversion of ethylbenzene and 93% selectivity to
acetophenone product, along with a minor (7%) 1-phenylethanol byproduct
(Table 2). On the other hand, the same reaction with only one equivalent
of TBHP also produced a very similar selectivity (93%) of acetophenone
and 7% of 1-phenylethanol byproduct, despite this reaction gave a lower
(51%) conversion of ethylbenzene compared to the reaction with two
equivalents of TBHP. The fact that both reactions, at lower and higher
TBHP, gave acetophenone (1) and 1-phenylethanol (2) in about similar
proportions starting in the early period of the reactions, regardless of
the amount of TBHP used, suggests that both 1 and 2 form in parallel via
two different mechanisms (instead of the formation of 2 first, followed
by its conversion into 1).

[0078] Thus, without being bound by theory, the mechanism of Au/SBA-15
catalyzed alkane oxidation probably starts with Au nanoparticle catalyzed
decomposition of TBHP (t-BuOOH) into t-BuOO. or t-BuO. radical species.
This will be followed by two different TBHP-catalyzed oxidation
reactions, producing ketones and secondary alcohols, from the alkene. The
t-BuOH is not expected to deactivate the Au nanocatalysts. In fact,
alcohols are sometimes used as solvents for Au-catalyzed oxidation
reactions of other substances such as alkanes as shown in Table 5 or even
oxidation of other alcohols (Mitsudome et al. (2009) Adv. Synth. Catal.,
351:1890-1896). Thus, the t-BuOH byproduct from the oxidation reaction
would not deactivate the catalyst; however, if it were used as a solvent
in larger quantity, it may lower the Au nanoparticles' catalytic activity
compared to other solvents, as shown in Table 5, but not when formed as a
byproduct.

[0079] In conclusion, mesoporous silica-supported nanosized Au particles
(Au/SBA-15) have been synthesized. Their use as efficient and selective
catalysts for oxidation of ethylbenzene, various other alkyl-substituted
benzenes, and different n-alkanes has been demonstrated. The Au/SBA-15
catalysts were shown to oxidize these reactants with TBHP as oxidant and
give predominantly the corresponding ketones under the reaction
conditions employed. Interestingly, the Au/SBA-15 catalysts generated the
ketone products selectively without requiring additives such as
carboxylic acids, which are often used for favoring selective oxidation
of alkanes into ketone products. The Au/SBA-15 materials were also shown
to be versatile selective oxidation catalysts as they successfully
catalyzed a series of other alkyl-substituted benzenes such as
propylbenzene and diphenylmethane, yielding their corresponding ketone
products with high conversion and selectivity. The Au/SBA-15 catalysts
also catalyzed n-alkanes including n-hexane and n-hexadecane, resulting
in unprecedented higher selectivities to their corresponding ketone
products. In addition, the Au/SBA-15 catalyst gave good catalytic
activities and very good selectivities in at least three catalytic
cycles.

[0080] While certain of the preferred embodiments of the present invention
have been described and specifically exemplified above, it is not
intended that the invention be limited to such embodiments. Various
modifications may be made thereto without departing from the scope and
spirit of the present invention, as set forth in the following claims.